arm64 UEFI early FDT handling

(+ Grant)

Quoted 10 lines · Ard Biesheuvel

This is a followup to the "arm64: update/clarify/relax Image and FDT placement rules" series I sent a while ago:
(http://article.gmane.org/gmane.linux.ports.arm.kernel/407148)

This has now been split in two series: this first series deals with the early FDT handling, primarily in the context of UEFI, but not exclusively.

A number of minor issues exist in the early UEFI/FDT handling path, such as:
- when booting via UEFI, memreserve entries are removed from the device tree but the /reserved-memory node is not

After reading Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt again, I think simply ignoring the reserved-memory node is not the way to go. The reason is that it may contain dynamic allocations that are referenced by other nodes in the DT, and there is no good technical reason IMO to disallow those. OTOH, static allocations may conflict with the UEFI memory map, so those need to be dropped or at least checked against the memory map. The problem here is that static nodes may also be referenced by phandle, so we need to handle the referring node in some way as well.

So I think we have a number of options:
- ignore /memreserve/s and reject static allocations in /reserved-memory (*) but honor dynamic ones
- ignore /memreserve/s and honor all of /reserved-memory after checking that static allocations don't conflict
- honor all /memreserve/s and /reserved-memory nodes and check all for conflicts
- ...

(*) static allocations for regions that the UEFI memory map does not describe should be OK, though

I personally prefer the first one, since a dynamic allocation implicitly conveys that the region does not contain anything special when coming out of boot, and there is very little we need to do other than perform the actual reservation. Static allocations are ambiguous in the sense that there is no annotation that explains the choice of address.

Thoughts, please?